Process for operating heating devices for liquid mixtures having components which react during evaporation with the formation of undesired products

ABSTRACT

A PROCESS FOR HEATING LIQUID MIXTURES HAVING COMPONENTS WHICH REACT TO FORM UNDESIRED PRODUCTS DURING HEATING. THE MIXTURE IS HEATED BY APPLICATION OF HEAT THROUGH HEATING SURFACES IN CONTACT WITH THE LIQUID MIXTURE. THE TEMPERATURE OF THE HEATING SURFACES IS MAINTAINED AT A PREDETERMINED DIFFERENTIAL RELATIVE TO THE BOILING POINT OF THE MIXTURE, TO LIMIT THE PRODUCTION OF UNDESIRED PRODUCTS TO A MINIMUM DURING HEATING.

Feb. 16,: 1.971 i-l. BALDUS ET AL 3,563,881 PROCESS FOR OPERATINGHEATING DEVICES FOR LIQUID MIXTURES ORATION WITH ION OF UNDESIREDPRODUCTS HAVINGCOMPONENTS WHICH REACT DURING EVAP THE; FORMAT 3Sheets-Sheet 1 Filed July 23, I968 SSDR RURE 0 DAT TLHU NANE EBA v HmMsR ENE BAT m n Nu w O\| plmo 7 3k Q Q Feb. 16, 1971 aALDus ETA!- I3,563,881 PROCESS FOR OPERATING HEATING DEVICES FOR LIQUID MIXTURESHAVING. COMPONENTS WHICH REACT DURING EVAPORATION WITH Filed July 23 THEFORMATION OF UNDESIRED PRODUCTS 5 Sheets-Sheet 2 l/ 1 M 1/- i .1 l 09,4/ l I l 0 MV- m A 02 wa /w, d ///QI n v. m l mw A m w w w x 0, w my...5

- W des INVENTORS HERBERT BALDUS mus MANHARD WALTER KREUTER ATTORNEY 3Sheets-sheaf; 3

-30 2:0 5 I 4 A 3 518 36 75.50 4 2; Ems: E5 525 Feb. 16, 1971 BALDUS ETAL PROCESS FOR OPERATI NQ HEATING DEVICES FOR LIQUID MIXTURES HAVINGCOMPONENTS -'wHIcH REACT DURING EVAPORATION WITH A A THE FORMATION OFUNDESIRED PRODUCTS Filed July 25 1968 INVENTORS HERBERT BALDUS HANSMANHARD WALTER KREUTER ATTORNEY United States Patent "O US. Cl. 208--48Claims ABSTRACT OF THE DISCLOSURE A process for heating liquid mixtureshaving components which react to form undesired products during heating.The mixture is heated by application of heat through heating surfaces incontact with the liquid mixture. The temperature of the heating surfacesis maintained at a predetermined differential relative to the boilingpoint of the mixture, to limit the production of undesired products to aminimum during heating.

BACKGROUND OF THE INVENTION This invention relates generally toprocesses for heating liquid mixtures and more particularly to processesfor heating liquid mixtures having components which react during heatingwith the formation of undesired products.

As an example of a mixture producing undesired products during heating,unpretreated heavy condensates of cracking gases, mainly C4-, C andcracked gasoline fractions, contain substances, such as butadiene,cyclopentadiene and styrene, which tend to form during heating, latexorcork-like polymerizates. These polymerizates form deposits in feedconduits and on heating surfaces of forced circulation evaporators. Thedistillation columns therefore must be provided with interchangeableevaporators having removable plates, so that, upon interchange, theplates can be removed from the down evaporator and the depositsmechanically removed therefrom. In case of forced circulationevaporators, shutdown and cleaning is normally required after anoperating period of a few weeks.

Another example of liquid mixtures yielding undesired products duringheating is the regeneration of dior triethylene glycol to remove watertherefrom. In such a case, if the evaporator temperature is notregulated properly, a reaction takes place in which two moles of glycolare condensed, giving rise not only to a loss in glycol, but also to theproduction of more water.

SUMMARY OF THE INVENTION An object of this invention, therefore is toprovide a method for heating liquid mixtures, wherein undesired productsare formed during evaporation, which overcomes the problems of the priorart by inhibiting such formation. Another object is to provide a methodof increasing the efiiciency and/or extending the operating time of suchheating devices with an attendant saving in cost and equipment.

These objects are attained, in accordance with this invention, bymaintaining the heating surfaces of the liquid mixture heating apparatusat predetermined desired temperature relative to the boiling temperatureof the liquid being heated. The desired temperature diflerential is afunction of the energy of activation of the reaction which forms theundesired products.

These and other objects of the invention will become 3,563,881 PatentedFeb. 16, 1971 better understood to those skilled in the art by referenceto the following detailed description when viewed in light of theaccompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a non-dimensional graph ofthe amount of undesired products formed per unit time as a function oftemperature differential between the heating surface and the liquidmixture; and

FIG. 2 is a plot of the desired heating surface temperature and theheating surface temperature difierential relative to the temperature ofthe liquid mixture for liquid mixtures of different boiling points as afunction of the energy of activation for the reaction which forms theundesired products.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with thisinvention, it has been discovered that in heating liquid mixtures of thetype described there is a relationship between the temperaturedifierential between the heating surface and the boiling point of theliquid mixture and the rate of production of undesired products, whichrelationship is a function of the energy of activation of the reactionwhich forms the undesired products.

This relationship is thought to derive from the followingconsiderations:

(1) M fc'-,dV [kmol/seo] wherein: (I)

M is the quantity of undesired products, formed per unit time in aheating device;

' c, is the speed of formation of the undesired products;

and V is the volume of the heating device.

In case of the customarily employed tubular evaporators with closegrouping of the heating surface, a simplification can be made asfollows:

wherein: imv) represents the speed of formation of the undesiredproducts at a constant temperature T of the heating surfaces.

Equation 1 reveals that M increases with the increasing volume V as longas e, and T remain unchanged.

(II) The volume V is, however, proportional to the area of the heatingsurface, H which latter depends in turn, where a specific amount of heatQ is to be transmitted, only on the diiference between the temperatureof the heating surface T and the boiling temperature of'the liquid, T,as follows:

Q HE a T T) wherein w=heat transfer coefficient.

Since the heating surface H varies with the volume by a factor ofproportionality s=V /H the following applies:

VH4 flaw-T As Equation 1' demonstrates, M increases with V V however, isdirectly proportional to the heating surface H and inverselyproportional to the temperature difference T T. M therefore increaseswith decreasing temperature differential T T, as long as the dependenceof the reaction speed c, on the temperature can be neglected.

(III) The speed of formation of the undesired products 6,, however,increases with increasing temperature T in accordance with the Arrheniusequation:

(3) kmol 111. sec.

E=energy of activation of the reaction forming the undesired products;

R=general gas constant;

k =a constant (experimentally determined; see John H. Perry, ChemicalEngineers Handbook," Third Ed, McGraw-Hill Book Co. Inc., 1950, pp.323-324).

E can be predicted in simple reactions by reference to standardhandbooks; however, in more complicated reactions, other means ofdetermination must be referred to. In heating processes where, forexample, polymerization is to be avoided, the determination of E causesparticular difficulties since the composition of the polymerizates andthe existence of numerous reaction stages, namely starting reactions,growth reactions and chain breaking reactions, cannot be accuratelydetermined due to the multitude of monomers present in these mixtures.

During the evaluation of results in connection with plants for suchprocesses, it has been found, however, that the important reaction inthe encrustation of the heating surfaces is in the starting reaction ofpolymerization, namely the radical formation by dissociation (i.e. (CHCOOC(CH 2(CH C-O- or CH =CHCH=CH 2CH CH-) with its attendant high energyof activation. The energy of activation of the dissociation ofdi-tert.-butyl peroxide (CH COO C(CH is, for example, on the order ofmagnitude of 36,000 kcal./kmol. In this reaction, the controlling valueof B then has been found to be 36,000 kcaL/kmol (see Frost and Pearson,-Kinetik und Mechanismen homogener chcmischer Reaktionen, Verlag Chemie,Weinheim/Bergstrasse, 1964, pp. 340-348).

In addition to polymerization reactions, condensation reactions can alsolead to the formation of undesired products. During the regeneration ofdior triethylene glycol which has absorbed water white being used as adrying agent and from which the water must be driven out by heating, twomolecules of the glycol are chemically added to each other, with theevolution of water. Although no solid products result from thisreaction, the consequences are a loss in glycol, as well as thereformation of water. The quantity of these undesired products wherein(see Equation 2), is balanced by the increase of due to the increasingspeed of formation of the deposits caused by the increasing heatingsurface temperature T can likewise be maintained at a minimum if theevaporator temperature is regulated in accordance with this e BTW k/oQSWlllh C a It has been found that this function has a pronounced minimumfor:

( it-MR Wdos T E or, in terms of T 4!! when E Wdes R 2 a wherein Twdesis the desired heating surface temperature for production of minimumdeposits.

Cir

In FIG. 1, a dimensionless representation of Equation 4 for constantevaporation temperature is illustrated,

wherein M/Ili is plotted against In this connection M is obtained byinserting the expression for TWdes from Equation 4 into Equation 4.

From FIG. 1, it can be seen how the energy of activation influencesdeviation from T For values of the abscissa above 1, M/M increases morerapidly with increasing ratio of the energy of activation E to thereference value E (assuming E, to be 10,000 kcal./kmol).

From FIG. 2, the values for T T (ordinate) and Twdes (abscissa) areplotted as related in accordance with Equation 4', to a given boilingtemperature T and a given energy of activation E. Using the numericalvalues R=1,986 kcal./kmol K. and E=35,000 kcal./kmo], the desired rangeis obtained from Equation 4":

Wdes This range is characterized in FIG. 2 by the shaded area. AlthoughTWdes is indicated as 1, for better re sults it is 10.5, more preferably:0.2. On the other hand, Twdes can also be as high as :L3, or higher,while obtaining substantial benefits of the invention.

If the mixture to be rectified boils, for example, at 373 K., and thechemical reaction which is primarily responsible for formation of theundesired products has an energy of activation E of 40,000 kcaL/kmol,then, from Equation 4 or from FIG. 2, T -T is 7.2 C. and TwdeS is 380.2K. In order to restrict deposits to a minimum, the heating surface ofthe evaporator must be maintained at the temperature of 380.2 K.

The evaporators used in carrying out the invention are described inStandards of Tubular Exchanger Manufactures Association, 4th ed., 1959,especially types AEL and BEM on p. 2. The materials are listed on p. 16(copper and copper alloys, carbon steel, aluminium and aluminiumalloys).

The difference between the temperature of the heating surface, T and theboiling temperature of the liquid, T, until now has usually been kept atabout 4060 C., in order to have a high heat transfer velocity, that isto keep small the heating surface of the evaporator.

The mean boiling temperature of a liquid mixture is the mean value ofthe boiling temperatures on the upper level and on the bottom of thatliquid mixture in the evaporator.

The effectiveness of prior art evaporators presently in service can alsobe determined from FIGS. 1 and 2. In order to explain this feature, thefollowing example is set forth: Assume that the mixture to be evaporatedhas an average boiling temperature T of 393 K., an energy of activationof the reaction casuing the formation of the undesired products E of35,000 kcal./kmol, and a temperature of the heating surface T of 415 K.From 5 FIG. 2, T -T is therefore 9.0" and TWdes is 402 K., as indicatedby dashed line. If the following ratio T T 415393 Wdea results, then itcan be seen from FIG. 1, as indicated by dashed line, that thearnount ofdeposits actually formed is approximately 1.6 times greater than theamount which would result at a heating surface temperature of 402 K.

Accordingly, with the aid of this invention, the amount of undesiredproducts formed per unit of time can be restricted to a minimum bysetting the desired temperature TWdes for the heating surface of thedevice. If evaporators, for example, are operated in accordance withthis invention, the operating time can be increased by a multiple, andin many cases, cleaning is required only on the occasion of a generalshutdown of the entire plant, for example, every one to two years, sothat auxiliary evaporators are dispensed with.

Examples for calculation of evaporators according to the invention:

(1) LIQUID MD(TURE.-DEPROPANIZER du'=1100 kcal./m. h. o C. ,:3000kcal./m. h. c.

(2) LIQUID MIXTURE-LOW PRESSURE STRIPPER BOTTOMS Weight percent 0 H,14.11 C4H8 34.68 C4H10 3.19 0 14.14 c 23.86 c /c 10.01 9+ 0.01 p =7 ata.T2=348 K.

:700 kcaL/m. h. C. a,, =3000 kcaL/m. h. C.

The values of the heat transfer coefiicient oz for a given evaporatorand liquid are tabulated in VDI Warmeatlas Berechnungsbl'aitter fiir denWarmeiibergang, VDI Verlag Diisseldorf.

In case of both examples the reaction causing polymerisation is assumedto be the formation of radicals from peroxides (see col. 5); the energyof activation therefore is E=35,000 kcaL/kmol. As discussed in col. 8,the termperature TWdes is calculated from Equation 4 or FIG. 2respectively:

In order to calculate the temperature of the heating steam now, theboiling temperature of the liquid, T, and the temperature of the heatingsurface, T are plotted against l/a, l/a (inner side, tube) and 1/0:(outer side,'shell) extending in opposite directions from the wall, asis seen from FIG. 3. The wanted value, T that is the correspondingcondensation temperature of the heating steam, is given by the point ofintersection of the l/a ordinate with the straight line through T and TIt is seen that stI= stz= 56 K. The preceding examples can be repeatedwith similar success by substituting the generically or specifically de-6 scribed reactants and/or operating conditions of this invention forthose used in the preceding examples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Consequently, such changes and modifications are properly,equitably, and intended to be, within the full range of equivalence ofthe following claims.

What is claimed is:

1. In a process for heating a liquid mixture having components whichreact to form undesired products during heating by application of heatthrough a heating surface in contact with the mixture, the improvementwherein said heating surface is maintained at a temperature TWGIesaccording to the following relationship wherein T=mean boilingtemperature in K. of the liquid mixture;

R egeneral gas constant;

E=energy of activation of the reaction forming the undesired products;and

X=a permissible deviation of not more than 3 K.

2. In a process of distilling unpretreated heavy condensates of crackinggas in a distillation column provided with an evaporator, theimprovement comprising the step of minimizing the polymerization ofconstituents of the condensates of the gases by maintaining the heatingsurfaces of said evaporator at a temperature 'I in accordance with thefollowing relationship wherein T=mean boiling temperature in K. of theliquid mixture; R=general gas constant; -E:energy of activation of thereaction forming the undesired products; and X=a permissible deviationof not more than 3 K. 3. A process as described by claim 1 wherein saidliquid mixture is depropanizer bottoms.

4. A process as described by claim 1 wherein said liquid mixture is lowpressure stripper bottoms.

5. A process as described by claim 1 wherein X is :1 K.

6. A process as described by claim 1 wherein X is i0.5 K.

7. A process as described by claim 1 wherein X is $0.2 K.

8. A process as described by claim 2 wherein X is i0.2 K.

9. A process as described by claim 3 wherein X is 2,668,793 2/1954Holland 196-116 DELBERT E. GANTZ, Primary Examiner GEORGE E. SCHMITKONS,Assistant Examiner U.S.Cl.X.R.

